The power output rating of dynamoelectric machines, such as large turbo-generators, is often limited by the ability to provide additional current through the rotor field winding because of temperature limitations imposed on the electrical conductor insulation. Effective cooling of the rotor winding contributes directly to the output capability of the machine. This is especially true in the rotor end region, where direct, forced cooling is difficult and expensive. As prevailing market trends require higher efficiency and reliability in lower cost, higher-power generators, cooling the rotor end region becomes a limiting factor.
Turbo-generator rotors typically have concentric rectangular coils mounted in slots in a rotor. The end portions of the coils (commonly referred to as “endwindings”) that extend beyond the main rotor body are typically supported against rotational forces by a retaining ring. (See FIG. 1). Support blocks are placed intermittently between the concentric coil endwindings so as to maintain their relative positions and to add mechanical stability for axial loads such as thermal loads. (See FIG. 2). Additionally, the copper coils are constrained radially by the retaining ring about an outer radius so as to counteract the centrifugal forces.
The presence of the spaceblocks and the retaining rings results in a number of coolant regions exposed to the copper coils. The primary coolant path extends axially between the spindle and the bottom of the endwindings. Discrete cavities are formed between the coils by the bounding surfaces of the coils, the blocks, and the inner surface of the retaining ring structure. The endwindings are exposed to the coolant that is driven by the rotational forces from radially below the endwindings into these cavities. (See FIG. 3). According to computed flow pathlines, this heat transfer tends to be low because the coolant flow enters the cavity, traverses through a primary circulation path, and then exits the cavity. This circulation path results in low heat transfer coefficients especially near the center of the cavity. Thus, this means for heat removal about the endwindings it is relatively inefficient.
Various schemes have been used to route additional cooling gas through the rotor end region. These cooling schemes rely on either (1) making cooling passages directly in the copper conductors by machining grooves or forming channels in the conductors and then pumping the gas to some other region of the machine and/or (2) creating regions of relatively higher and lower pressures with the addition of baffles, flow channels, and pumping elements to force the cooling gas to pass over the conductor surfaces.
Some systems penetrate the highly stressed rotor retaining ring with radial holes so as to allow cooling gas to be pumped directly alongside the rotor endwindings and discharged into the air gap. Such systems, however, have only limited usefulness due to the high mechanical stress and fatigue considerations relating to the retaining ring.
If the conventional forced rotor end cooling schemes are used, considerable complexity and cost are added to rotor construction. For example, directly cooled conductors must be machined or fabricated to form the cooling passages. In addition, an exit manifold must be provided to discharge the gas somewhere in the rotor. The forced cooling schemes require the rotor end region to be divided into separate pressure zones, with the addition of numerous baffles, flow channels, and pumping elements. These elements again add complexity and cost.
If the forced or direct cooling schemes are not used, then the rotor endwindings should be cooled passively. Passive cooling relies on the centrifugal and rotational forces of the rotor to circulate gas in the blind, dead-end cavities formed between concentric rotor windings. Passive cooling of rotor endwindings is sometimes also called “free convection” cooling.
Although passive cooling provides the advantage of minimum complexity and cost, heat removal capability may be diminished when compared to the active systems of direct and forced cooling. Any cooling gas entering the cavities between the concentric rotor windings must exit through the same opening because these cavities are otherwise enclosed. The four “side walls” of a typical cavity are formed by the concentric conductors and the insulating blocks that separate them with the “bottom” (radially outward) wall formed by the retaining ring that supports the endwindings against rotation. Cooling gas enters from the annular space between the conductors and the rotor spindle. Heat removal is thus limited by the low circulation velocity of the gas in the cavity and the limited amount of the gas that can enter and leave these spaces.
In typical configurations, the cooling gas in the end region has not yet been fully accelerated to the rotor speed. As the fluid is driven into a cavity, the heat transfer coefficient is typically highest near the spaceblock that is downstream relative to the flow direction, i.e., where the fluid enters with the highest momentum and where the fluid coolant is the coldest. The heat transfer coefficient also is typically high around the cavity periphery while the center of the cavity receives the least amount of cooling.
Increasing the heat removal capability of the passive cooling systems will increase the current carrying capability of the rotor. This increase capability may provide increased rating capability of the generator as a whole while maintaining the advantage of low cost, simple, and reliable construction.